In a control system, a voltage driver is used to energize an external system, such as a relay, contactor or solenoid valve. When used to drive an external load, the driver typically needs to protect itself and its upstream source from the possibility of a short-circuit or load impedance fault to either the load or its cable connections. In some circumstances the load may also need to be protected against excess voltage being passed to it from the upstream source. Protection against short-circuit faults is usually implemented by way of a fuse, thermal breaker, over-current trip or current limit.
Existing voltage drivers utilize a pass transistor operated as a switch (fully on or fully off) during normal operation. The pass transistors are further controlled in a linear mode when a current limited mode of operation is desired. While in the linear mode, the pass transistor passes current while dropping a portion of the supply voltage across the input and output terminal of the pass transistor, in order to limit the current through the pass transistor to a maximum value. The drop in supply voltage induces power dissipation within the pass transistor. In alternative systems, where an overvoltage protection system is implemented alongside the pass transistor, the pass transistor acts as a linear voltage regulator and again dissipates a large amount of power due to the transistor being placed in the linear mode.
During the linear mode, the pass transistor dissipates a large amount of power. The large amount of power dissipation requires that the pass transistor be sized sufficient to handle the power dissipation. Such a transistor, however, is oversized for standard operations resulting in an increased cost and weight. Further, continued dissipation of the high amounts of power within the pass transistor over time can damage the pass transistor. To mitigate this effect the pass transistor is required to be turned off in a short time period after being placed in the linear mode. As such, continued linear mode operations, such as voltage and current limiting, cannot be maintained for an extended period.